1. Technical Field
The present invention relates generally to gas turbine engine cooling flows and, more specifically, to modulating cooling flows for turbine components and for airframe heat loads.
2. Background Information
A typical gas turbine engine of the turbofan type generally includes a forward fan and a booster or low pressure compressor, a middle core engine, and a low pressure turbine which powers the fan and booster or low pressure compressor. The core engine includes a high pressure compressor, a combustor and a high pressure turbine (HPT) in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are connected by a high pressure shaft. High pressure air from the high pressure compressor is mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the high pressure compressor.
The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine (LPT). The low pressure turbine extracts energy from the gas stream for rotatably driving the fan and booster compressor via a low pressure shaft. The low pressure shaft extends through the high pressure rotor. Most of the thrust produced is generated by the fan. Marine or industrial gas turbine engines have low pressure turbines which power generators, ship propellers, pumps and other devices while turboprops engines use low pressure turbines to power propellers usually through a gearbox.
Since the HPT is subject to the hottest combustion gases discharged from the combustor, various components thereof are typically cooled by bleeding a portion of the pressurized air from the compressor. Any air used for turbine cooling is lost from the combustion cycle and, therefore, reduces overall efficiency of the engine. Each turbine stage includes a row of turbine rotor blades extending radially outwardly from a supporting rotor disk with the radially outer tips of the blades being mounted inside a surrounding turbine shroud. Typically turbine rotor blades of at least the first turbine stage are cooled by the bled portion of the pressurized air from the compressor.
The typical turbofan aircraft engine initially operates at a low power, idle mode and then undergoes an increase in power for takeoff and climb operation. Upon reaching cruise at the desired altitude of flight, the engine is operated at lower, or intermediate power setting. The engine is also operated at lower power as the aircraft descends from altitude and lands on the runway, following which thrust reverse operation is typically employed with the engine again operated at high power. In the various transient modes of operation of the engine where the power increases or decreases, the turbine blades heat up and cool down respectively.
The HPT blades are typically cooled using a portion of high pressure compressor discharge air bled (also known as compressor discharge pressure or CDP air) from the last stage of the compressor. The air is suitably channeled through internal cooling channels inside the hollow blades and discharged through the blades in various rows of film cooling holes from the leading edge and aft therefrom, and also typically including a row of trailing edge outlet holes or slots on the airfoil pressure side. This blade cooling air bypasses the combustion process and therefore further reduces efficiency of the engine.
Blade cooling air is gathered and transferred from static portions of the engine to rotating disks supporting the hollow blades. In order to efficiently transfer the blade cooling air, tangential flow inducers have been designed, usually in the form of a circumferentially disposed array of nozzles to accelerate and turn the cooling flow so as to tangentially inject the cooling flow into rotating rotors at a rotational or tangential speed and direction substantially equal to that of the rotor. Each inducer injects the cooling air flow in a direction that is tangent to the operational direction of rotation of the rotor at an exit hole or orifice at the downstream or aft end of the inducer.
One method to reduce cooling is to cutback cooling flow at low power settings thereby improving efficiency. The traditional approach for modulating physical airflow through an orifice or passage is by increasing and decreasing the flow area at an orifice.
Accordingly, it is desired to provide a gas turbine engine having improved blade cooling control and efficiency.